In order to meet with the requirements of LTE-Advanced systems and to enhance the performance of the conventional systems, a series of new techniques, such as relay, carrier aggregation, and CoMP (Coordinated Multi-Point) transmission and reception have been proposed. Among the newly proposed techniques, the CoMP (generally referred to as Co-MIMO, collaborative MIMO, network MIMO, etc.) is being highly appraised as a technique that can enhance communication performance of a cell boundary user equipment and increase a throughput of a sector. Generally, in an multiple cell environment, wherein a frequency re-usage rate is equal to 1, inter-cell interference may degrade the performance of the user equipments and may also decrease the throughput of the corresponding sector. A simple manual method for reducing the inter-cell interference (i.e., an FFR (Fractional Frequency Reuse) method of a user equipment (UE)-specific power control) is applied in the LTE systems in order to enhance the throughput of a cell boundary user equipment within an interference-limited environment. Instead of reducing the usage of frequency resource in each cell, it is more preferable to reduce inter-cell interference by re-using inter-cell interference by means of a desirable signal. In order to achieve this object, a plurality of CoMP (Coordinated Multi-Point) methods has been proposed. Hereinafter, a CoMP system will hereinafter be described briefly.
A CoMP system refers to a system for enhancing the throughput of a user located within a cell boundary by applying an enhanced MIMO (Multiple-Input Multiple-Output) transmission in a multiple cell environment. By adopting the CoMP system, the Inter-Cell Interference within a multiple cell environment may be reduced. By using such CoMP system, a user equipment may be commonly supported with data from a Multi-cell base station.
Also, by simultaneously supporting one or more user equipments (UE 1, UE 2, . . . UE K) using the same radio frequency resource, each base station may enhance the system performance. Also, based upon channel state information (CSI) between the base station and the user equipments, the base station may perform an SDMA (Space Division Multiple Access) method.
The above-described CoMP method may be divided into a JP (Joint Processing) method of a coordinated MIMO (Co-MIMO) format via data sharing and a CS/CB (Coordinated Scheduling scheme/Beamforming scheme).
FIG. 1 illustrates a conceptual view of CoMP operations of an intra base station (intra eNB) and an inter base station (inter eNB).
Referring to FIG. 1, intra base stations (110, 120) and an inter base station (130) exist in a Multi Cell environment. In an LTE (Long Term Evolution), an intra base station is configured of several cells (or sectors). Cells belonging to a base station, to which a specific user equipment belongs, are related with the specific user equipment and the intra base stations (110, 120). More specifically, cells sharing the same base station as the user equipments belonging to the cells are referred to as the cells corresponding to the intra base stations (110, 120), and the cells belonging to other base stations may be referred to as cells corresponding to the inter base station (130). As described above, although cells that are based upon the same base station as the specific user equipment transmit and receive information (e.g., data, CSI (Channel State Information)) to and from one another through an x2 interface, cells that are based upon another base station may transmit and receive inter-cell information to and from one another via Backhaul (140).
As shown in FIG. 1, a single-cell MIMO user (150) located within a single cell may communicate with a single serving base station within a single cell (sector), and a multi-cell MIMO user (160) located at a cell boundary may communicate with multiple serving base stations within multiple cells (sectors).
In order to perform such CoMP operations, the user equipment is required to feed-back the CSI to the serving base station. Among the CSI, PMI, RI may be used in all CoMP operation methods, and PMI, RI formats may be used in all CoMP operation methods. However, among the CSI within an LTE system, it may be difficult to apply the format of the CQI in all CoMP operation methods without any modification. Therefore, as a new measurement is required to be performed in order to allow the user equipment to feed-back CQI values of neighboring cells, which perform CoMP operations, a problem of having a large overhead with respect to the measurement may occur.